For DC circuits, DC means Direct Current. A direct current circuit is a path for charged electrons to travel through. DC circuits must have a source of energy (a battery), resistors (lightbulbs) and conductors (wires). There are three types of DC circuits: series, parallel, and combination.
Series
In a series circuit, the total resistance is equal to the sum of the values of the resistance provided by each resistor, and the total voltage is the same across each resistor. However, the voltage drop in the entire circuit is equal to the sum of the voltage dropped across each resistor. The current is also the same at any point in the wires all the way around the circuit, and the total current is equal to the voltage divided by the total resistance. If a lightbulb is removed from a series circuit, the rest of the lightbulbs go out because without every lightbulb, the cicuit is broken.
Parallel
In a parallel circuit, the voltage across each branch of the cicuit is equal to the voltage across the other branch of a circuit. The total resistance of a parallel circuit is found by adding the reciprocals of the total resistance in each arm of the circuit and finding the reciprocal of the result. The total current in a parallel circuit is equal to the voltage of the battery divided by the total resistance, so the amount current in each branch is inversely proportional to the amount of resistance. If you removed one of the lightbulbs in a parallel circuit, the other lightbulb stays lit because the circuit isn't broken, so there is still a path for the current.
Complex
Complex circuits are a combination of series and parallel circuits.The total resistance is found by adding the resistance in the parallel circuit to the resistance of the two lightbulbs that are in series. The total current is found by dividing the batteries' voltage by the total resistance. The two lighbulbs in series are the brightest because they have the most current flowing through them. The lightbulb in the branch of the parallel circuit by itself is the second brightest becuase the same amount of current flows through each branch of the parallel circuit, but it has half the resistance of the other branch of the parallel circuit because the resistance of the branch with two lightbulbs is twice the resistance of the branch with only one lightbulb. The reason the branch of the parallel circuit with two lightbulbs has more resistance is greater is because the two lightbulbs in the branch is equal to the sum of the resistance of two lightbulbs. If I removed one of the lightbulbs that is in series, the entire circuit would go out because the circuit would be broken. If I removed the lightbulb from the branch of the parallel circuit with only one lightbulb, all of the others would stay on, because there is another path for the current to flow through. If I removed one of the lightbulbs from the branch of the parallel circuit with two lightbulbs, the other lightbulb in the branch would go out because they are in series, but the other lightbulbs would stay on, because the circuit wouldn't be broken.
